Where did the dominiques come from. One source I have found to have high frequency of single combs.It can vary, I find about 1 in 30 or so. The ones who usually have the single combs are the roosters, though I've had hens with single combs too. I do not use them in my breeding program.
They are still Dominiques, just with a defect. It is like an orpington with pink legs (instead of white), or a polish without a crest. They are still that breed, just defective specimens.